Stabilization of rubber



1 3,061,586 STABILIZATION F RUBBER Ralph B. Thompson, Hinsdale, 11].,assignor to Universal Oil Products Company, Des Plaines, 111., acorporation of Delaware No Drawing. Filed Dec. 30, 1959, Ser. No.862,758

13 Claims. (Cl. 260-4595) ThlS 1s a continuation-in-part of applicationSerial No.

707,693, filed January 8, 1958, now Patent No. 3,017,422, and relates tothe stabilization of rubber against cracking due to ozone. The presenceof ozone in the atmosphere appears to be mcreasing, particularly incertain localities. This increase in ozone concentration in theatmosphere results in an increase in the ozone cracking of rubber. Thepresent lnvention is directed to a novel method of retarding and/orpreventing such cracking.

The present invention is particularly applicable to the stabilization ofnatural rubber. Natural rubber generally is regarded as comprisingnaturally occurring isoprene polymers. The natural rubbers include Hevearubber, caoutchouc, balata, gutta-percha, etc. Although natural rubberappears to have better tear resistance during normal service thansynthetic rubber, the natural rubber does undergo ozone cracking and, inunusual service or long periods of' service, does tail of ozonecracking. Furthermore, the presence of cracks in rubber products also isobjectionable for aesthetic reasons. Customers and users of rubberproducts object to the unsightly cracks in the rubber and, therefore, itis important that such isobutylene (butyl rubber), etc. Still othersynthetic rubbers include Thiokolrubber, silicone rubber, neoprenerubber, etc.

In one embodiment, the present invention relates to a method ofpreventing the cracking of rubber due to ozone which comprisesincorporating therein a stabilizing concentration of an additive of thefollowing general structure:

where R R R R R and R are selected from the group consisting of hydrogenand alkyl groups and X is selected from the group consisting of oxygenand sulfur.

The inhibitor for use in the present invention is prepared by thereaction of a quinone methide with a phos- 3,061,586 Patented Oct. 30,1962 phoric acid ordithiophosphoric acid, and may be illustrated by thefollowing general equation:

in which the symbols R through R have the same significance as indicatedabove.

Any suitable quinone methide may be used in preparing the inhibitor.While these compounds may be named as quinone methides, they also may benamed as 4-alkylidene-2,5 -cyclohexadiene-1-one. Thus, for example, acompound in which R and R are tertiary butyl groups and R and R aremethyl groups may be named 2,6-di-tertbutyll-isopropylidene quinonemethide. This compound also may be named 2,6di-tert-butyl-4-isopropylidene-2,5- cyclohexadiene-l-one. The latterterminology appears preferred and, accordingly, will be used in thepresent specification and claims.

The preferred quinone methide for use as a reactant in the presentinvention comprises one in which R and R are tertiary alkyl groups and Rand R are alkyl groups. Where R and R are methyl groups, preferredquinone methides include2,6-di-tert-butyl-4-isopropylidene-2,S-cyclohexadiene-l-one,2,6-di-tert-amyl-4-isopropy1idene-2,5- cyclohexadiene-l-one,2,6-di-tert-heXy1-4-isopropylidene-2, S-cyclohexadiene 1 one,2,6-di-tert-heptyl-4-isopropylidene-2,5-cyclohexadiene-l-one,2,6-di-tert-octyl-4-isopropylidene-Z,S-cyclohexadiene-l-one, etc. Ingeneral it is preferred that both tertiary alkyl groups are the samealthough, in some cases, they may be different as, for example, incompounds including 2-tert-butyl-6-amyl-4-isopropylidene 2,5cyclohexadiene-l-one, etc., 2-tert-amyl-6-tert-hexyl-4-isopropylidene-2,S-cyclohexadiene-l-one, etc. Ashereinbefore set forth, it is preferred that R and R are tertiary alkylgroups. However, in some cases, these ,may be secondary or primary alkylgroups and will includes compounds as2,6-diisopropyl4-isopropylidene-2,5- cyclohexadiene-l-one,2,6-di-sec-butyl-4-isopropylidene-Z, 5 cyclohexadiene-l-one,2,6-dimethyl-4-isopropylidene-2, 5 cycloheXadiene-l-one,2,6-diethyl-4-isopropylidene-Z,5- cyclohexadiene -1 -one,Z-methyl-6-tert-butyl-4-isopropylidene-Z,S-cyclohexadiene-l-one,2-methyl-6-sec-butyl-4-isopropylidene-Z,S-cyclohexadiene-l-one,2-methyl-6-n-butyl- 4-isopropylidene-2,S-cyclohexadiene-l-one,Z-methyl-G-isopropyl-4-isopropylidene-2,5-cyclohexadiene-Lone, etc.

While quinone methides having an isopropylidene group in the 4-positionare preferred, it is understood that compounds having other alkylidenegroups may be employed including, for example, such groups asmethylidene, ethylidene, l-propylidene, butylidene, pentylidene,hexylidene, heptylidene, oetylidene, etc. It is understood that thevarious quinone methides which may be used as a reactant are notnecessarily equivalent and also that two or more quinone methides may beemployed.

Any suitable phosphoric acid or thiophosphoric acid and preferably alkylsubstituted acids may be employed in preparing the inhibitor of thepresent invention. Alkyl and preferably dialkyl dithiophosphoric acidsare employed. Dialkyl dithiophosphoric acids include diisopropyldithiophosphoric acid, dibutyl dithiophosphoric acid, diamyldithiophosphoric acid, dihexyl dithiophosphoric acid, diheptyldithiophosphoric acid, dioctyl dithiophosphoric acid, dinonyldithiophosphoric acid, didecyl dithiophosphoric acid, diundecyldithiophosphoric acid, didodecyl dithiophosphoric acid, ditridecyldithiophosphoric acid, ditetradecyl dithiophosphoric acid, dipentadecyldithiophosphoric acid, dihexadecyl dithiophosphoric acid, diheptadecyldithiophosphoric acid, dioctadecyl dithiophosphoric acid, dinonadecyldithiophosphoric acid, dieicosyl dithiophosphoric acid, etc.Conveniently, these alkyl groups are introduced through the use of fattyalcohols and thus the alkyl groups may be selected from capryl, lauryl,myristyl, palmityl, stearyl, cetyl, etc. It is understood that the alkylgroups may be straight or branched chain, that the alkyl groups may beprimary, secondary or tertiary substituents, and that R and R maycomprise the same or different alkyl groups. Monoalkyl dithiophosphoricacids, when employed, preferably comprise those in which the alkyl groupis selected from those hereinbefore set forth.

In another embodiment of the invention alkyl or dialkyl phosphoric acidsare utilized in preparing the inhibitor of the present invention.Illustrative alkyl phosphoric acids include isopropyl phosphoric acid,diisopropyl phosphoric acid, butyl phosphoric acid, dibutyl phosphoricacid, pentyl phosphoric acid, dipentyl phosphoric acid, hexyl phosphoricacid, dihexyl phosphoric acid, heptyl phosphoric acid, diheptylphosphoric acid, octyl phosphoric acid, dioctyl phosphoric acid, nonylphosphoric acid, dinonyl phosphoric acid, decyl phosphoric acid, didecylphosphoric acid, undecyl phosphoric acid, diundecyl phosphoric acid,dodecyl phosphoric acid, didodecyl phosphoric acid, tridecyl phosphoricacid, ditridecyl phosphoric acid, pentadecyl phosphoric acid,dipentadecyl phosphoric acid, hexadecyl phosphoric acid, dihexadecylphosphoric acid, heptadecyl phosphoric acid, diheptadecyl phosphoricacid, octadecyl phosphoric acid, dioctadecyl phosphoric acid, nonadecylphosphoric acid, dinonadecyl phosphoric acid, eicosyl phosphoric acid,dieico'syl phosphoric acid, etc. Here again, it is understood that, inthe dialkyl phosphoric acids, the alkyl groups may be the same ordifferent.

When desired, a mixture of the monoand/ or dialkyl phosphoric acidsand/or monoand/or dialkyl dithiophosphoric acid may be employed. In somecases these are recovered as a mixture and may be used in the presentinvention without the added cost of separating the individual compounds.For example, a mixture of monoand dioctyl phosphoric acids is availablecommercially, generally at a lower cost than the individual compounds,and may be used in the present invention. It is understood that thevarious phosphates which may be used in preparing the inhibitor of thepresent invention are not necessarily equivalent but generally Will beselected with reference to the specific rubber in which it is to beused.

Referring to the general structure hereinbefore set forth, it will benoted that the final product contains an aromatic ring. The substitutedquinone aromatizes readily and thus the final product Will contain thearomatic configuration. Accordingly, the reaction of the presentinvention may be considered as an addition-rearrangement type.

The reaction of the quinone methide and phosphoric or thiophosphoricacid is effected in any suitable manner. The reaction is exothermic andpreferably is controlled by effecting the same in the presence of aninert solvent. Any suitable solvent may be employed, an aromatichydrocarbon being particularly preferred. The aromatic hydrocarbonsinclude benzene, toluene, xylene,

ethylbenzene, cumene, etc. Other solvents include saturated aliphaticesters, as ethyl acetate, amyl acetate, 2- ethylhexyl acetate, methylpropionate, methyl butyrate, ethyl butyrate, isopropyl butyrate, etc.,saturated aliphatic nitriles as acetonitrile, propionitrile, etc.,dioxane, nitrobenzene, chlorobenzene, chloroform, carbon tetrachloride,etc. The specific temperature of reaction will depend upon whether asolvent is employed and, when used, upon the particular solvent. Ingeneral, the temperature may range from about -20 C. to about C. and insome cases up to C., although temperatures outside of this range may beemployed, depending upon the specific reactants and solvents utilized.

The reaction normally may be effected in the absence of a catalyst. Insome cases, it is preferred to use a catalyst, and any suitable catalystmay be employed. Illustrative catalysts include anhydrous hydrogenchloride, p-toluene sulfonic acid, piperidine, etc.

From the equation hereinbefore set forth, it will be noted that one moleequivalent of quinone methide reacts with one mole equivalent ofphosphoric acid or thiophosphoric acid. As hereinbefore set forth, amixture of quinone methide or a mixture of phosphoric acids may beemployed. The product will contain a mixture of compounds. However, formost cases, the mixed product is satisfactory for use as an inhibitor inrubber and thus may be so employed without'incurring the additionalexpense of separating individual compounds.

The inhibitors of the present invention are quinone methide phosphatesand quinone methide dithiophosphates. Thus the reaction may be describedas the addi tion of one mole of a phosphoric acid, preferably a dialkylphosphoric acid, or one mole of a dithiophosphoric acid, preferably adialkyl dithiophosphoric acid, to one mole of a4-alkylidene-2,5-cyclohexadiene-l-one. A specific illustration is theaddition of diisopropyl dithiophosphoric acid to2,6-di-tert-butyl-4-isopropylidene-2,S-cyclohexadiene-l-one to prepareS-3,5-di-tert-butyl-4-hydroXy-alpha, alpha-dimethylbenzyl 0,0diisopropyl dithiophosphate. Other specific preferred compounds includeS-3,5-di-tertpentyl-4-hydroxy-alpha,alpha-dimethylbenzyl 0,0 diisopropyldithiophosphate, S-3,5-di-tert hexyl 4hydroxyalpha,alpha-0,0-diisopropyl dithiophosphate,S-3,5-di-tertheptyl-4-hydroxy-alpha,alpha-0,0-diisopropy1dithiophosphate,S-3,5-di-tert-octyl-4-hydroxy-alpha,alpha-0,0-diisopropyldithiophosphate, etc.,S-3,5-di-tert-butyl-4-hydroxyalpha-methyl-alpha-ethylbenzyl 0,0diisopropyl dithiophosphate, S-3,5-di-tert-butyl 4hydroxy-alpha,alpha-diethyl-0,0-diisopropyl dithiophosphate,S-3,5-di-tert-butyl- 4-hydroxy-alpha-methyl-alpha-propyl-0,0-diisopropyldithiophosphate,S-3,5-di-tert-butyl-4-hydroxy-alpha-alphadipropyl-0,0-diisopropyldithiophosphate, etc.,S-3,5-ditert-butyl-4-hydroxy-alpha,alpha-dimethylbenzyl-0,0 dibutyldithiophosphate, S-3,S-di-tert-butyl-4-hydroxy-alpha,alpha-dimethylbenzyl-0,0-di-pentyl dithiophosphate, 8-3,5-di-tert-butyl-4-hydroxy-alpha,a1pha-dimethylbenzyl-0,0- di-hexyldithiophosphate, S-3,5-di-tert-butyl-4-hydroxyalpha,alpha-dimethylbenzyl0,0 di heptyl dithiophosphate,S-3,5-di-tert-butyl-4-hydroxy-alpha,alpha-dimethylbenzyl-0,0-di-octyldithiophosphate, S-3,5-di-tert-butyl-4-hydroxy-alpha,alpha-dimethylbenzyl-0,0-di-lauryl dithiophosphate,S-3,5-di-tert-butyl 4 hydroxy-alpha,alpha-dimethylbenzyl-0,0-di-stearyldithiophosphate, etc. It is understood that these specific compounds arelisted for illustrative purposes only and are not intended to limit thescope of the present invention thereto.

The inhibitor of the present invention is recovered as a viscous liquidor solid. It may be marketed as such or as a solution in a suitablesolvent including, for example, saturated parafinic hydrocarbonsincluding pentane, hexane, heptane, octane, etc., aromatic hydrocarbonsincluding benzene, toluene, xylene, cumene, etc., alcohols, ketones,etc.

The concentration of inhibitor will depend upon the particular rubber inwhich it is to be used. In general the inhibitor will be utilized in aconcentration of from about 0.25% to about and more particularly fromabout 2% to about 5% by weight of the rubber hydrocarbon. Theseconcentrations are based on the rubber hydrocarbon, exclusive of theother components of the final rubber composition, and are used in thismanner in the present specification and claims. It is understood thatthe inhibitor of the present invention is utilizable along with otheradditives incorporated in rubber for specific purposes including, forexample, antioxidants, accelerators, softeners, extenders, Wax,reinforcing agents, etc.

The inhibitor of the present invention also possesses antioxidantproperties and, therefore, will serve to prevent both cracking due toozone and deterioration due to oxygen. However, in many cases, it isdesirable to employ a separate antioxidant and, in such cases, theinhibitor of the present invention is employed along with a separateantioxidant. Any suitable antioxidant may be employed including, forexample, phenyl-beta-naphthylamine, 6-phenyl-2,2,4 trimethyl 1,2dihydroquinoline, marketed under the trade name of Santoflex-B, 2,2-methylene-bis-(4-methyl-6-tert-butyl-phenol), 2,6-di-tertbutyl-p-cresol,the reaction product of acetone and diphenylamine, marketed under thetrade name of B.L.E., etc. These antioxidants generally are used in aconcentration from about 0.5% to about 3% by weight of the rubber.

The inhibitor of the present invention also normally is employed alongwith paraflin and/or micro-crystalline wax. The wax generally isutilized in a concentration of from about 0.5% to about 3% by weight ofthe rubber.

In one embodiment, the inhibitor of the present invention is admixedwith the antioxidant and/or wax, and the mixture then is composited withone or more of the other components of the rubber composition.

The inhibitor of the present invention is incorporated in rubber orrubbery products in any suitable manner and at any suitable stage ofpreparation. When the inhibitor is added to a liquid, such as rubberpigment or an oil, it is dissolved therein in the desired proportions.When it is to be added to a solid substrate, it is incorporated there inby milling, mastication, etc. The additive may be utilized as such or asa solution or dispersion, or as a paste, etc.

It is understood that the inhibitor can be utilized in any rubbercomposition subject to ozone cracking, including those used for rubbertires and tubes, hose, belts, sheet and thread rubber, rubberizedfabrics, molded goods, boots and shoes, etc., whether vulcanized in amold, in open steam, in hot air or in the cold by the so-called acidprocess. Furthermore, it is understood that the inhibitor of the presentinvention can be used in reclaims and latices of rubbery materials,whether or not admixed with fillers, pigments, accelerating agents, etc.In another embodiment the present invention can be utilized for thestabilization of adhesives, elastomers, etc. which tend to crack due toozone.

The following examples are introduced to illustrate further the noveltyand utility of the present invention but not withthe intention of undulylimiting the same.

Example I liquid and is S-3,5-di-tert-butyl-4 hydroxy alpha,alpha- Idimethylbenzyl- 0,0-diisopropyl dithiophosphate. The

6 calculated analysis for sulfur and phosphorus are 13.91% and 6.74%,respectively. Actually found are 12.90% sulfur and 6.65% phosphorus. 1

Example 11 The inhibitor prepared according to Example I was evaluatedas an antiozonant in white rubber. A sample of the rubber without thisadditive and a sample of the rubber containing one percent by Weight ofthis additive each were subjected to evaluation in an ozone cabinet. Thesamples were elongated 20% and mounted on a board and then were exposedto an atmosphere containing 25 parts of ozone per million parts of airat 100 F. The sample of rubber without additive showed cracks within 4hours of exposure in the ozone cabinet. 6n the other hand, the sample ofrubber containing the additive was substantially free from cracks after24 hours of exposure in the ozone cabinet. Furthermore, this additiveaccelerated curing of the rubber and, therefore, the amount ofaccelerating agent normally added to rubber may be decreasedaccordingly.

Example III The dialkyl dithiophosphoric acid used in this example iscapryl stearyl dithiophosphoric acid. An equal molar equivalent of thisdialkyl dithiophosphoric acid was reacted with an equal molar equivalentof 2,6-di-tertbutyl-4-isopropylidene-2,5 cyclohexadiene-l-one insubstantially the same manner as described in Example I. The product isS-3,5-di-tert-butyl-4-hydroxy-alpha,alphadirnethylbenzyl-O-caprylO-stearyl dithiophosphate and was recovered as a yellow-green liquid.This product is incorporated in black natural rubber in a concentrationof 2% by weight of the rubber hydrocarbon and serves to retard crackingof the rubber due to ozone.

Example IV Equal molar quantities of2,6-di-tert-butyl-4-methylidene-2,5-cyclohexadiene-l-one and a mixedmonoand dioctyl orthophosphoric acid Were dissolved in a hexane solventand the solution was saturated with anhydrous hydrogen chloride.Following the initial reaction, the reaction mixture was heated for 4hours on a steam bath to complete the reaction, after which the solventwas removed by distillation under vacuum. The product is a mixture ofO-3,5-di-tert-butyl-4-hydroxy-alpha,a1-pha-dimethylbenzyl-O-octyl-phosphate and O-3,5-di-tertbutyl-4 hydroxyalpha,alpha dimethylbenzyl-0,0-dioctyl phosphate. The individualphosphates may be recovered from the mixture if desired. However, ashereinbefore set forth, the mixture may be used as such and thisexpensive and complicated separation may be avoided. The mixtureprepared in the above manner is utilized in a concentration of 3% byweight'in GR-S rubber and serves to retard cracking thereof due toozone.

I claim as my invention:

1. Natural rubber normally subject to cracking due to ozone containingfrom about 0.25% to about 10% by weight of an inhibitor of the followinggeneral structure:

where R R R R, R and R are selected from the group consisting ofhydrogen and alkyl groups and X is selected from the group consisting ofoxygen and sulfur. 2. Natural rubber, normally subject to cracking dueto ozone containing from about 0.25% to about 10% by Weight ofS-3,5-di-tert-butyl-4-hydroxy-alpha,alphadimethylbenzyl-0,0-diisopropyldithiophosphate.

3. Natural rubber, normally subject to cracking due to ozone containingfrom about 0.25% to about 10% by Weight of S-3,5-di tert-butyl4-hydroxy-alpha,alpha-dimethylbenzyl-O-capryl-O-stearyl dithiophosphate.

4. Natural rubber normally subject to cracking due 5 to ozone containingfrom about 0.25% to about 10% by weight ofS-3,5-di-tert-butyl-4-hydroxy-alpha,alphadimethylbenzyl-0,0-dioctyldithiophosphate.

5. Natural rubber normally subject to cracking due to ozone containingfrom about 0.25% to about 10% by 10 weight ofO-3,5-di-tert-butyl-4-hydroxy-alpha,alpha-dimethylbenzyol-0,0-dioctylphosphate.

6. Natural rubber subject to cracking due to ozone containing from about0.25% to about 10% by weight of O-3,5-di-tert-butyl 4hydroxy-alpha,alpha-dimethyl- 15 benzyl-0,0-diisophopy1 phosphate.

7. A rubber normally subject to cracking due to ozone containing fromabout 0.25% to about 10% by weight of an inhibitor having the followinggeneral structure:

where R R R R R and R are selected from 3 the group consisting ofhydrogen and alkyl g oups and X is selected from the group consisting ofoxygen and sulfur.

8. A rubber normally subject to cracking due to ozone containing fromabout 0.25% to about 10% by weight of S-3,5 di-tert-butyl 4 hydroxyalpha,alpha dimethylbenzyl diisopropyl dithiophosphate.

9. A rubber normally subject to cracking due to ozone containing fromabout 0.25% to about 10% by weight of S 3,5-di-tert-butyl 4hydroxy-alpha,alpha-dimethyl- 'benzyl-O-capryl-O-stearyldithiophosphate.

10. A rubber normally subject to cracking due to ozone containing fromabout 0.25% to about 10% by Weight of S-3,5-di-tert-butyl4-hydroXy-alpha,alpha-dimethylbenzyl-0,0-dioctyl phosphate.

11. A rubber normally subject to cracking due to ozone containing fromabout 0.25 to about 10% by weight ofO-3,5-di-tert-butyl-4-hydroXy-alpha,alpha-dimethylbenzyl-0,0-dioctylphosphate.

12. A rubber normally subject to cracking due to ozone containing fromabout 0.25% to about 10% by weight of O-3,5-di-tert-butyl 4hydroxy-alpha,alpha-dimethyl-benzyl-0,0-diisopropyl phosphate.

13. A composition as defined in claim 7 further characterized in thatsaid rubber is a co-polymer of butadiene and styrene.

References Cited in the file of this patent UNITED STATES PATENTS2,415,833 Mikeska et al Feb. 18, 1947 2,530,339 Mikeska et a1. Nov. 14,1950 2,587,477 Hunter Feb. 26, 1952 2,589,675 Cook et a1. Mar. 18, 1952

7.A RUBBER NORMALLY SSUBJECT TO CRACKING DUE TO OZONE CONTAINING FROMABOUT 0.25% TO ABOUT 10% BY WEIGHT OF AN INHIBITOR HAVING THE FOLLOWINGGENERAL STRUCTURE: WHERE R1, R2, R3, R4, R5 AND R6 ARE SELECTED FROM THEGROUP CONSISTING OF HYDROGEN AND ALKYL GROUPS AND X IS SELECTED FROM THEGROUP CONSISTING OF OXYGEN AND SULFUR.